In telecommunication, telephony is the term for the general use of equipment to provide voice communication over distances, specifically by connecting telephones to each other. Telephones originally were connected directly together in pairs. Each user had separate telephones wired to the various places they might wish to reach. With the invention of the telephone exchange, each telephone could be connected to other local ones and eventually far away phones.
In modern systems, equipment at the exchange connects a telephone line to another in the same wire center, or to a trunk at a distant exchange when the user of a telephone wants to make a telephone call. The exchanges together form the Public Switched Telephone Network (“PSTN”). Digital telephony is the use of digital technology in the provision of telephone services and systems. Almost all telephone calls are provided this way, but sometimes the term is restricted to cases in which the last mile is digital or where the conversion between digital and analog signals takes place inside the telephone.
IP Telephony is a modern form of telephony which uses the TCP/IP protocol popularized by the internet to transmit digitized voice data. Computer Telephony Integration (“CTI”) enables computers to know about and control phone functions such as making and receiving voice, fax, and data calls with telephone directory services and caller identification. The integration of telephone software and computer systems is a major development in the evolution of the automated office. With the proliferation of various types of telephony systems, it is not uncommon to have two or more independent systems exist side-by-side in a location.
Users of two or more independent systems may experience cross ringing where a single incoming call is directed to multiple end devices (or even client applications). For example, a call from an external network such as a PSTN may be directed to a user's desktop device and mobile device on a Voice Over IP (VOIP) network. Similarly, calls from a mobile network (e.g. a cellular network) may also be connected to the user through the mobile network directly or through a combination of the mobile network with the VOIP network.